Traditional integrated circuit (IC) sockets are generally constructed of an injection molded plastic insulator housing which has stamped and formed copper alloy contact members stitched or inserted into designated positions within the housing. The designated positions in the insulator housing are typically shaped to accept and retain the contact members. The assembled socket body is then generally processed through a reflow oven which melts and attaches solder balls to the base of the contact member. During final assembly, the socket can be mounted onto a printed circuit assembly. The printed circuit assembly may be a printed circuit board (PCB), the desired interconnect positions on the PCB are printed with solder paste or flux and the socket is placed such that the solder balls on the socket contacts land onto the target pads on the PCB. The assembly is then reheated to reflow the solder balls on the socket assembly. When the solder cools it essentially welds the socket contacts to the PCB, creating the electrical path for signal and power interaction with the system.
During use, the socket receives one or more IC packages and connects each terminal on the IC package to the corresponding terminal on the PCB. The terminals on the IC package are held against the contact members by applying a load to the package, which is expected to maintain intimate contact and reliable circuit connection throughout the life of the system. No permanent connection is required so that the IC package can be removed or replaced without the need for reflowing solder connections.
These types of sockets and interconnects have been produced in high volume for many years. As systems advance to next generation architectures, these traditional devices have reached mechanical and electrical limitations that mandate alternate approaches.
As processors and electrical systems evolve, several factors have impacted the design of traditional sockets. Increased terminal count, reductions in the terminal pitch (i.e., the distance between the contacts), and signal integrity have been main drivers that impact the socket and contact design. As terminal count increases, the IC packages get larger due to the additional space needed for the terminals. As the IC package grows larger the relative flatness of the IC package and corresponding PCB becomes more important. A certain degree of compliance is required between the contacts and the terminal pads to accommodate the topography differences and maintain reliable connections.
IC package manufacturers tend to drive the terminal pitch smaller so they can reduce the size of the IC package and reduce the flatness effects. As the terminal pitch reduces, however, the surface area available to place a contact is also reduced, which limits the space available to locate a spring or a contact member that can deflect without touching a neighbor.
In order to maximize the length of the spring so that it can deflect the proper amount without damage, the thickness of the insulating walls within the plastic housing is reduced. Thinner walls increase the difficulty of molding as well as the latent stress in the molded housing that can cause warping due to heat applied during solder reflow.
For mechanical reasons, longer contact members traditionally have been preferred because they have desirable spring properties. Long contact members, however, tend to reduce the electrical performance of the connection by creating a parasitic effect that impacts the signal as it travels through the contact. Other factors, such as contact resistance, impact self heating as current passes through, for example, power delivery contacts. Also, the small space between contact members can cause distortion as a nearby contact member influences a neighboring contact member, which is known as cross talk.
Traditional sockets and methods of fabricating the same are able to meet the mechanical compliance requirements of today's needs, but they have reached an electrical performance limit. Next generation systems will operate above 5 GHz and beyond and the existing interconnects will not achieve acceptable performance levels without significant revision.